1. Field of the Invention
This invention relates to a manufacturing method for a screw or threaded member. In particular, this invention pertains to a self-tapping and self-drilling screw which is made of austenitic stainless steel and carburized steel.
2. Prior Art
Due to the rapid development and research of zinc and aluminum coatings of steel sheet, the generally accepted ten year operational life of these kinds of steels has been extended to approximately twenty years. Protective coatings or corrosion resistant layers applied to steel structures and screw retaining elements are also being rapidly developed and studied. Self-tapping and self-drilling screws for fixing coated steel sheet is easily damaged and deformed due to the fact that they are exposed to the external environment. Recently, the constant increased requirements for construction quality has brought a large demand for self-tapping and self-drilling screws made of austenitic stainless steel. However, due to the fact that the metallurgical and mechanical properties of austenitic stainless steel is such that it is easily hardened and poorly shaped when it is formed and is further difficult to meet the requirements of self-drilling by nitriding processes. There are a number of unsolved problems in the conventional manufacturing method for self-tapping and self-drilling screws of austenitic stainless steel which can penetrate 5 mm thickness low carbon steel plate. Some of the problems are as follows:
1. Prior art self-tapping screws of austenitic stainless steel treated by nitriding is shown in FIG. 1 which is a schematic view of a conventional self-tapping stainless steel screw. The portion of the screw designated as N in solid black line in FIG. 1 is the nitride area. The screw is composed of austenitic stainless steel head 1 and a self-tapping thread portion 2. Generally, the surface hardness of the threaded portion can be improved by a nitriding treatment. Although the surface hardness of threaded portions can be improved by nitriding, this conventional screw has the following drawbacks:
(a) the corrosion-resistance of the surface of austenitic stainless steel screws are degraded due to the composition of the nitride; and, PA1 (b) nitriding temperature of austenitic steel is about 600.degree. C. lasting for 2-4 hours which causes crystal and grain boundary corrosion problems to the stainless steel and reduces the overall corrosion-resistant function. PA1 (a) High carbon steel is not conducive to forging. Such is suited to shaping the drilling end portion through a machine cut which raises the manufacturing costs. On the other hand, the hardness of heat-effected zones influenced by welding in the high carbon steel side is over HV500 degrees, so it tends to induce cracking and become difficult to thread and further shortens the life of threading dies; and, PA1 (b) After the self-tapping and drilling end sections of high carbon steel are quenched, they are fully sectionally hardened. This makes the fully sectional hardness of tapping and drilling end sections reach as high as HV550-HV600 degrees so that both sections become brittle and frangible.
2. Prior art self-tapping and drilling screws of austenitic stainless steel may be sectionally hardened but are frangible. In such prior art screws, in order to accomplish self-tapping and self-drilling functions, the screw is formed from different materials into separate portions which include the tapping and drilling portion, and the jointing portion. These two portions are welded together, shaped, threaded and quenched. As shown in FIG. 2, which is a schematic view of a self-tapping and drilling screw which are formed of austenitic stainless steel and high carbon steel, the screw consists of a jointing portion including a stainless steel head 1, threaded bolt 2, and a drilling portion including self-tapping section 3, as well as a drilling end section 4 which are formed of high carbon steel. The manufacturing method is to weld the material of the jointing and drilling portions at the area W and then cut and thread the drill and tap portion. Subsequently, they are quenched to increase the fully sectional hardness of the self-tapping section 3 and the drilling end section 4. Although the screw is made of high carbon steel and is quenched to improve the fully sectional hardness for the self-tapping section 3 and drilling end section 4, such still has the following drawbacks:
3. Prior art self-tapping and drilling screws of austenitic stainless steel are contaminated by carbon. In order to overcome the drawbacks of high hardness of a heat effected zone after being influenced by welding and the drawback of being easily cracked or frangible, the manufacturing method is to use a low carbon steel for the material of the self-tapping and drilling section which is welded to the jointing portion of the stainless steel and then carburized and quenched together. As is shown in FIG. 3, another schematic of the self-tapping and drilling screw which is made of austenitic stainless steel wherein the black line C in FIG. 3 is the carburized area, the screw consists of a jointing portion including a stainless steel head 1, threaded bolt 2, and a drilling portion including self-tapping section 3 as well as a drilling end section 4 which are formed of low carbon steel. The manufacturing method is to weld materials of the stainless steel and low carbon steel portions at the area W and then thread and form the end portion. Subsequently, they are carburized and quenched to improve the surface hardness for the self-tapping section 3 and drilling end section 4. Although this method can solve the welding problems of high carbon steel joints, the austenitic stainless steel head and threaded bolt are contaminated by carbon due to the carburization step. Such has the drawbacks of diminishing the original corrosion-resistant function. Today, emphasis is placed on development of corrosion-resistance and work quality, thus this method of manufacture is less used than the above-described high carbon steel self-tapping and drilling screws.